1. Field of the Invention
The present invention relates to an optical modulation system, and more particularly, to the interconnection of external electrodes to an optical modulator so as to minimize loss of signal energy and to prevent the introduction of spurious modes into the signal within the optical modulator.
2. Discussion of the Related Art
In a general fiber optical communication system, optical signals are sent along an optical fiber communication line to a desired location. Optical modulators with performance in the 40 GHz frequency range and beyond, are critical components in optical communication systems.
To achieve high-frequency operation in a material such as LiNbO3, the electrical and optical velocity of the modulating and modulated signal must be matched. This is achieved by employing thick ( greater than 10 xcexcm) electrodes in conjunction with a buffer layer (typically SiO2). The buffer layer is deposited directly on the LiNbO3 and the electrode structure is delineated on the buffer layer. While the buffer layer facilitates velocity matching, it also results in decreased modulation efficiency because the applied voltage is partially dropped across the buffer layer. LiNbO3 is an anisotropic material, with the following dielectric constants:
xcex5extra-ordinary=26,xcex5ordinary=43
Thus, planar and uni-planar transmission lines such as microstrip, coplanar waveguide (CPW) and coplanar strip (CPS) tend to be very dispersive when built directly on LiNbO3. As the frequency increases, the fields become more concentrated in the regions below the metal strips, where the substrate permittivity has already resulted in a relatively larger electric displacement since the fields are forced into the LiNbO3 to an increasing extent as the frequency increases. Therefore, a frequency-dependent effective permittivity can be defined for the transmission line.
FIG. 1 illustrates an optical modulator of the prior art. The modulator has a mounting base 1 that is typically conductive or made of a non-conductive material covered with a conductive layer. The mounting base 1 is typically at the ground potential of the device and will herein be referred to as the grounded base 1. The optical modulator has an optical modulator chip 2, for example a LiNbO3 chip covered with an insulating buffer layer, mounted on the grounded base 1. The grounded base 1 includes input/output optical terminals 345 and input/output electrical terminals 71. The optical modulator chip 2 has two ground electrodes 3/3xe2x80x2 and a signal electrode 4 mounted on top of the buffer layer above a waveguide 34 of the optical modulator chip 2. This electrode configuration is known as the coplanar-waveguide (CPW). When the electrode structure of the optical modulator chip 2 comprises just one signal electrode, and one ground plane, it is known as the coplanar-strips (CPS) configuration.
The optical modulator chip 2 is comprised of an active section 6 and non-active sections 5. The active section 6 of the device is the section of the optical modulator chip 2 wherein the electrical and optical signals interact to cause optical modulation. Typically, the electrode dimensions, such as the width of the signal electrode 4, and the electrode gap tend to be very narrow (5-25 microns) in the active section 6. These dimensions are prohibitively small to facilitate direct connection of the device to standard electrical connectors. Hence, the electrodes 3/4/3xe2x80x2 for the active section 6 are flared 31/41/31xe2x80x2 in the non-active section 5 to facilitate external connection to the signal electrode line 4 and the ground electrodes 3/3xe2x80x2. The flared electrodes 31/41/31xe2x80x2 do not take part in the process of optical modulation, but are required to facilitate connection of the active section of the modulator to standard electrical interface media. External electrical connection to the flared electrodes 31/41/31xe2x80x2 of the optical modulator chip 2 is facilitated by either a transition chip 7 having leads 23/24/23xe2x80x2 connected to the flared electrodes 31/41/31xe2x80x2 of the optical modulator chip 2 via wires or a direct external connection to the flared electrodes 31/41/31xe2x80x2 of the optical modulator chip 2 via wires from the electrical terminals 71.
FIG. 2 illustrates a side view of the optical modulator in the direction shown as Axe2x80x94A in FIG. 1. FIG. 2 shows electrodes 31/41/31xe2x80x2 on a buffer layer 8 terminating on the top surface edge of the optical modulator chip 2 and the grounded base 1 underlying the optical modulator chip 2. Although the ground electrodes 31/31xe2x80x2 of FIGS. 1 and 2 are shown as single lines, the ground electrodes may be ground planes which cover most of the top surface of the optical modulator chip 2 except for the signal electrode 4 and an area just outside the signal electrode 4. For example, there can be ground planes that cover most of the top surface of the optical modulator chip 2 but are no closer to the signal electrode than the ground electrodes 3/3xe2x80x2 shown.
The intended electrical guided mode for an optical modulator contains the frequency of an input or frequencies of input on the optical modulator for operating the optical modulator. Typically, an optical modulator has a range of sets of frequencies that can be used as electrical inputs to modulate an optical signal. For proper operation of the modulator, the intended electrical guided mode of the device must be such that the electric fields originating from the signal electrode must properly terminate on the adjacent ground electrodes without straying elsewhere in the modulator chip or package. The intended electrical guided mode of the optical modulator will hereinafter be referred to as the dominant CPW mode of the optical modulator.
Once the electric fields of the signal electrode penetrate through the buffer layer into the optical modulator chip, several other effects could occur. Depending on frequency, a CPW mode may couple with other extraneous electrical modes that the structure of the optical modulator can support. These modes could either be highly dispersive slab modes, or could be zero-cut-off modes. Examples of extraneous modes are: transverse-electric (TE) or transverse magnetic slab modes, slot-line mode (that could occur between the two ground planes of the CPW structure), parallel-plate modes (that could be excited between the electrodes on the top surface and the grounded base), and microstrip mode (between the top electrodes and the grounded base). When coupling to extraneous modes occurs, there is a loss of power for the dominant CPW mode. Such a power loss degrades the optical modulator""s modulation performance and the clarity of the output modulated optical signal is degraded. The amount of power lost to spurious or other extraneous modes depends on the field overlap between the dominant CPW mode and the other extraneous modes supported by the device.
One approach to avoid coupling to spurious or other extraneous modes in CPW structures is by reducing the cross-sectional dimension of the CPW transmission line. Referring to FIG. 1, by decreasing (S+2D), which is the width of electrode 4 plus twice the distance that one of the optical modulator grounds 3/3xe2x80x2 is located from the signal electrode 4, there is less field penetration into the optical modulator chip 2 and hence less of an opportunity for overlap between the guided CPW mode and other extraneous modes that can be supported by the device. Since there is less overlap in structures with smaller (S+2D), between the CPW mode and other extraneous modes, there is less of a power loss from the CPW mode and hence less degradation of the outputted modulated optical signal.
However, a CPW transmission line with a smaller cross-sectional dimension is not very practical because the device still requires external electrical connection. Typically, in the nonactive sections 5 of the optical modulator chip, the electrodes 3/4/3xe2x80x2 for the active section 6 become respective flared electrodes 31/41/31xe2x80x2 to facilitate connection to the signal electrode line and the ground electrodes. The connection is facilitated by the use of a transition chip 7 having leads connected to the flared electrodes 31/41/31xe2x80x2 of the optical modulator chip 2 or a direct external connection to the flared electrodes 31/41/31xe2x80x2 of the optical modulator chip 2. Although FIG. 1 shows two transition chips 7, a single transition chip for external connection to the optical modulator chip can extend down the side of the optical modulator chip and contain both sets of the electrodes 31/41/31xe2x80x2. Alternately, the modulator electrode can also be terminated with an appropriate resistance or a resistance-capacitance combination at the end of the electrode. Due to the relatively wider dimensions of the flared electrodes 31/41/31xe2x80x2 in the non-active sections 5 compared to the electrodes 3/4/3xe2x80x2 in the active section 6, there is significant field penetration into the optical modulator chip 2 (i.e. LiNbO3) through the buffer layer 8 in the non-active sections 5. This penetration increases the opportunity for extraneous mode coupling into substrate slab modes or zero-cutoff modes that the structure (i.e. the optical modulator chip, the CPW transmission line and the grounded base) can support in both the active section 6 and non-active section 5.
Ground plane integrity between the ground and the signal electrode is important for satisfactory operation of the optical modulator. Otherwise, high-speed optical modulation in the active sections 6 of the optical modulator chip will be seriously hampered. This is because over the frequency range of interest, the electrical velocity and hence impedance varies at the input to the modulator (i.e. the flared electrodes), and coupling to spurious modes occurs. As a result, the optical modulation in the active section 6 will not be in concert with the inputted electrical signal to the optical modulator.
Accordingly, the present invention is directed to an optical modulator that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
The present invention provides an optical modulator with enhanced ground plane integrity to minimize loss of signal energy and to prevent the introduction of extraneous modes into the modulated optical signal.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described, an optical device of the present invention includes a grounded base; an optical modulator chip having a top surface, a back surface and side surfaces, wherein the optical modulator chip is positioned on the grounded base with the back surface of the optical modulator chip facing the grounded base; and a first ground electrode, a signal electrode and a second ground electrode located over the top surface of the optical modulator chip, wherein the first and second ground electrodes are respectively connected to first and second extended resistive electrodes that extend down at least one side of the optical modulator chip and connect to the grounded base.
In another aspect, an optical device of the present invention includes: a grounded base; a transition chip for interconnecting an optical modulator chip having a top surface, a back surface and side surfaces, wherein the transition chip is positioned on the grounded base with the back surface of the transition chip facing the grounded base; and a first ground connection lead, a signal connection lead and a second ground connection lead located over the top surface of the transition chip, wherein the first and second ground connection leads are interconnected to first and second extended resistive connection leads that extend down at least one side of the transition chip and connect to the grounded base.
In another aspect, an optical device of the present invention includes: a grounded base; an optical modulator chip positioned on the grounded base having a top surface, a back surface and side surfaces; a first ground electrode, a signal electrode and a second ground electrode located over the top surface of the optical modulator chip; a transition chip for interconnecting the optical modulator chip having a top surface, a back surface and side surfaces, wherein the transition chip is positioned on the grounded base with the back surface of the transition chip facing the grounded base; and a first ground connection lead, a signal connection lead and a second ground connection lead located on the top surface of the transition chip, wherein the first and second ground connection leads are connected to first and second extended resistive connection leads that extend down at least one side of the transition chip and connect to the grounded base.
In another aspect, an optical device of the present invention includes: a grounded base; an optical modulator chip having a top surface, a back surface and side surfaces, wherein the optical modulator chip is positioned on the grounded base with the back surface of the optical modulator chip facing the grounded base; at least one ground electrode and a signal electrode located over the top surface of the optical modulator chip, wherein the at least one ground electrode is connected to an extended resistive electrode that extends down one side of the optical modulator chip and connects to the grounded base; and a transition chip for interconnecting the optical modulator chip having a top surface, a back surface and side surfaces, wherein the transition chip is positioned on the grounded base with the back surface of the transition chip facing the grounded base.